An electrophotographic light-sensitive material may have various structures depending upon the characteristics required or an electrophotographic process being employed.
An electrophotographic system in which the light-sensitive material comprises a support having thereon at least one photoconductive layer and, if necessary, an insulating layer on the surface thereof is widely employed. The electrophotographic light-sensitive material comprising a support and at least one photoconductive layer formed thereon is used for the image formation by an ordinary electrophotographic process including electrostatic charging, imagewise exposure, development, and, if necessary, transfer.
Furthermore, a process of using an electrophotographic light-sensitive material as an offset master plate for direct plate making is widely practiced.
Binders which are used for forming the photoconductive layer of an electrophotographic light-sensitive material are required to be excellent in the film-forming property by themselves and the capability of dispersing a photoconductive powder therein. Also, the photoconductive layer formed using the binder is required to have satisfactory adhesion to a base material or support. Further, the photoconductive layer formed by using the binder is required to have various excellent electrostatic characteristics such as high charging capacity, small dark decay, large light decay, and less fatigue due to pre-exposure and also have an excellent image forming properties, and the photoconductive layer stably maintaining these electrostatic properties in spite of the change of humidity at the time of image formation.
Binder resins which have been conventionally used include silicone resins (e.g., JP-B-34-6670, the term "JP-B" as used herein means an "examined published Japanese patent application"), styrene-butadiene resins (e.g., JP-B-35-1960), alkyd resins, maleic acid resins, polyamides (e.g., JP-B-35-11219), polyvinyl acetate resins (e.g., JP-B-41-2425), vinyl acetate copolymers (e.g., JP-B-41-2426), acrylic resins (JP-B-35-11216), acrylic acid ester copolymers (e.g., JP-B-35-11219, JP-B-36-8510, and JP-B-41-13946), etc.
However, in the electrophotographic light-sensitive materials using these binder resins, there are various problems such as 1) the affinity of the binder with a photoconductive powder is poor thereby reducing the dispersibility of the coating composition containing them, 2) the charging property of the photoconductive layer containing the binder is low, 3) the quality (in particular, the dot image reproducibility and resolving power) of the image portions of duplicated images is poor, 4) the image quality is liable to be influenced by the environmental conditions (e.g., high temperature and high humidity or low temperature and low humidity) at the time of the formation of the duplicated image, and 5) the photoconductive layer is insufficient in film strength and adhesion, which causes, when the light-sensitive material is used for an offset master, peeling off of the photoconductive layer, etc. at offset printing to reduce the number of prints.
In order to improve electrostatic characteristics of the photoconductive layer, various attempts have hitherto been made. For example, incorporation of a compound having an aromatic ring or a furan ring containing a carboxy group or a nitro group either alone or in combination with a dicarboxylic anhydride in a photoconductive layer is disclosed in JP-B-42-6878 and JP-B-45-3073. However, the thus improved electrophotographic light-sensitive materials are yet insufficient in electrostatic characteristics and, in particular, light-sensitive materials having excellent light decay characteristics have not yet been obtained. Thus, for compensating the insufficient sensitivity of these light-sensitive materials, an attempt has been made to incorporate a large amount of a sensitizing dye into the photoconductive layer. However, light-sensitive materials containing a large amount of a sensitizing dye undergo considerable deterioration of whiteness to reduce the quality as a recording medium, sometimes causing deterioration in dark decay characteristics, whereby satisfactory reproduced images are not obtained.
On the other hand, JP-A-60-10254 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") discloses a method of using a binder resin for a photoconductive layer by controlling the average molecular weight of the resin. That is, JP-A-60-10254 discloses a technique for improving the electrostatic characteristics (in particular, reproducibility at repeated use as a PPC light-sensitive material), moisture resistance, etc., of the photoconductive layer by using an acrylic resin having an acid value of from 4 to 50 and an average molecular weight of from 1.times.10.sup.3 to 1.times.10.sup.4 and an acrylic resin having an acid value of from 4 to 50 and an average molecular weight of from 1.times.10.sup.4 to 2.times.10.sup.5 in combination.
Furthermore, lithographic printing plate precursors using electrophotographic light-sensitive materials have been extensively investigated and various binder resins for a photoconductive layer have been prepared as satisfying both the electrostatic characteristics as an electrophotographic light-sensitive material and the printing characteristics as a printing plate precursor. For example, JP-B-50-31011 discloses a combination of a resin having a molecular weight of from 1.8.times.10.sup.4 to 10.times.10.sup.4 and a glass transition point (Tg) of from 10.degree. to 80.degree. C. obtained by copolymerizing a (meth)acrylate monomer and other monomers in the presence of fumaric acid and a copolymer composed of a (meth)acrylate monomer and a copolymerizable monomer other than fumaric acid, JP-A-53-54027 discloses a terpolymer containing a (meth)acrylic acid ester unit with a substituent having a carboxylic acid group at least 7 atoms apart from the ester linkage, JP-A-54-20735 and JP-A-57-202544 disclose a tetra- or pentapolymer containing an acrylic acid unit and a hydroxyethyl (meth)acrylate unit, and JP-A-58-68046 discloses a terpolymer containing a (meth)acrylic ester unit with an alkyl group having from 6 to 12 carbon atoms as a substituent and a vinyl monomer containing a carboxyl group as effective for improving the oil-desensitization of the photoconductive layer.
However, none of these resins proposed have proved to be satisfactory for practical use in charging property, dark charge retention characteristic, photosensitivity, and smoothness of the photoconductive layer.
Also, as the result of evaluations on the conventional binder resins which are said to be developed for electrophotographic lithographic printing plate precursors, it has been found that they have problems in the above-described electrostatic characteristics, background stains of prints, etc.
For solving these problems, JP-A-63-217354 discloses that the smoothness and the electrostatic characteristics of a photoconductive layer can be improved and images having no background stains are obtained by using a low-molecular weight resin (molecular weight of from 1,000 to 10,000) containing from 0.05 to 10% by weight of a copolymerizable component having an acidic group in the side chain of the copolymer as the binder resin, JP-A-1-100554 discloses a binder further containing a curable group-containing copolymerizable component together with the above-described acidic group-containing copolymerizable component, JP-A-1-102573 discloses a binder resin using crosslinking agent together with the above-described acidic group-containing resin, JP-A-63-220149, JP-A-63-220148, and JP-A-64-564 disclose a binder resin using a high molecular weight resin having a weight average molecular weight of at least 1.times.10.sup..varies. in combination with the above-described acidic group-containing resin, and JP-A-1-102573 discloses a binder resin using a heat- and/or photo-curable resin in combination with the above-described acidic group-containing resin.
On the other hand, as other binder resins for electrophotographic light-sensitive materials for solving the above-described problems, JP-A-1-70761 discloses a binder resin using a resin having a weight average molecular weight of from 1.times.10.sup.3 to 1.times.10.sup.4 having an acidic group at the terminal of the polymer main chain, JP-A-1-214865 discloses a binder resin using the above-described resin further containing a curable group-containing component as a copolymerizable component, JP-A-2-874 discloses a binder using a cross-linking agent together with the above-described resin, JP-A-1-280761, JP-A-1-116643, and JP-A-1-169455 disclose a binder resin using a high molecular weight resin having a weight average molecular weight of at least 1.times.10.sup.4 in combination with the above-described resin, and JP-A-2-34859 discloses a binder resin using a heat- and photo-curable resin in combination with the above-described resin.
However, it has been found that these resins still have problems in maintenance of the stable high performance when the electrophotographic light-sensitive materials are exposed to noticeably severe conditions.
More specifically, it has been found that, when a charging speed is increased in a charging step of the light-sensitive material, uneven charging occurs, which results in causing unevenness in the duplicated images, or, when a duplicating operation is carried out directly after irradiating the surface of the electrophotographic light-sensitive material with a fluorescent lamp, etc., as a supplemental operation for a copying machine, the duplicated images obtained are deteriorated (in particular, lowering of the image density, lowering of the resolving power, and the occurrence of background fog) (so-called pre-exposure fatigue).
Furthermore, when the electrophotographic light-sensitive material described above is used as lithographic printing plate precursor by an electrophotographic system, the printing plate has the duplicated images having deteriorated image quality in the case of carrying out the duplication under the above-described condition, and, when printing is conducted using the plate, serious problems may occur such as lowering of image quality and the occurrence of background fog.